Support running an executable file when a new lease is acquired.
If no 'script-file = SCRIPTFILE' is specified in the configuration file and if no '-X SCRIPTFILE' or '--script-file SCRIPTFILE' command argument is provided, then this functionality is entirely inactive and no associated subprocess is spawned. Otherwise, ndhc will spawn a subprocess that runs as root that has the sole job of forking off a subprocess that exec's the specified script in a sanitized and fixed-state environment whenever a new DHCPv4 lease is acquired. Note that this script is provided no information about ndhc or the DHCP state in the environment or in any argument fields; it is the responsibility of this script to gather whatever information it needs from either the filesystem or syscalls. This design is intended to avoid the historical problems that are associated with dhcp clients invoking scripts. The path of the scriptfile cannot be changed after ndhc is initially run; ndhc forks off the privsep script subprocess that executes scripts after it has read the configuration file and command arguments, but before it begins processing network data; thus, it is impossible for the network-handling process to modify or influence the script assuming proper OS memory protection. The privsep channel communicates that the script should be run by simply writing a newline; anything else will result in ndhc terminating itself. Before the recommended way to update system state after a change in lease information was to run the fcactus program and watch the associated leasefile for the interface for modification; now no external program is needed for this job.
This commit is contained in:
Nicholas J. Kain
172
scriptd.c
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172
scriptd.c
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// Copyright 2022 Nicholas J. Kain <njkain at gmail dot com>
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// SPDX-License-Identifier: MIT
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#include <unistd.h>
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#include <stdlib.h>
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#include <string.h>
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#include <errno.h>
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#include <assert.h>
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#include <poll.h>
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#include <signal.h>
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#include <sys/wait.h>
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#include <sys/prctl.h>
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#include <sys/stat.h>
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#include "nk/log.h"
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#include "nk/io.h"
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#include "nk/exec.h"
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#include "scriptd.h"
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#include "ndhc.h"
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#include "sys.h"
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#define MAX_ENVBUF 2048
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#define MAX_CENV 50
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bool valid_script_file = false;
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// Runs the 'script_file'-specified script. Called from ndhc process.
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void request_scriptd_run(void)
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{
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if (!valid_script_file) return;
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static char buf[] = "\n";
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ssize_t r = safe_write(scriptdSock[0], buf, 1);
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if (r < 0 || (size_t)r != 1)
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suicide("%s: (%s) write failed: %zd", client_config.interface,
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__func__, r);
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}
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static void run_script(void)
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{
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char *env[MAX_CENV];
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char envbuf[MAX_ENVBUF];
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switch ((int)fork()) {
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case 0: {
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int r = nk_generate_env(0, NULL, NULL, env, MAX_CENV, envbuf, sizeof envbuf);
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if (r < 0) {
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static const char errstr[] = "exec: failed to generate environment - ";
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safe_write(STDERR_FILENO, errstr, sizeof errstr);
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static const char errstr0[] = "(?) unknown error";
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static const char errstr1[] = "(-1) account for uid does not exist";
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static const char errstr2[] = "(-2) not enough space in envbuf";
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static const char errstr3[] = "(-3) not enough space in env";
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static const char errstr4[] = "(-4) chdir to homedir or rootdir failed";
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switch (r) {
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default: safe_write(STDERR_FILENO, errstr0, sizeof errstr0); break;
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case -1: safe_write(STDERR_FILENO, errstr1, sizeof errstr1); break;
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case -2: safe_write(STDERR_FILENO, errstr2, sizeof errstr2); break;
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case -3: safe_write(STDERR_FILENO, errstr3, sizeof errstr3); break;
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case -4: safe_write(STDERR_FILENO, errstr4, sizeof errstr4); break;
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}
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safe_write(STDERR_FILENO, "\n", 1);
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exit(EXIT_FAILURE);
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}
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nk_execute(script_file, NULL, env);
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}
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case -1: {
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static const char errstr[] = "exec: fork failed\n";
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safe_write(STDERR_FILENO, errstr, sizeof errstr);
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exit(EXIT_FAILURE);
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}
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default: break;
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}
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}
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static void process_client_socket(void)
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{
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static char buf[32];
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static size_t buflen;
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if (buflen == sizeof buf)
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suicide("%s: (%s) receive buffer exhausted", client_config.interface,
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__func__);
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int r = safe_recv(scriptdSock[1], buf + buflen, sizeof buf - buflen,
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MSG_DONTWAIT);
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if (r == 0) {
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// Remote end hung up.
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exit(EXIT_SUCCESS);
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} else if (r < 0) {
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if (errno == EAGAIN || errno == EWOULDBLOCK)
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return;
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suicide("%s: (%s) error reading from ndhc -> scriptd socket: %s",
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client_config.interface, __func__, strerror(errno));
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}
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buflen += (size_t)r;
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if (buflen > 1 || buf[0] != '\n') exit(EXIT_SUCCESS);
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buflen = 0;
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run_script();
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}
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static void do_scriptd_work(void)
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{
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struct pollfd pfds[2] = {0};
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pfds[0].fd = scriptdSock[1];
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pfds[0].events = POLLIN|POLLHUP|POLLERR|POLLRDHUP;
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pfds[1].fd = scriptdStream[1];
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pfds[1].events = POLLHUP|POLLERR|POLLRDHUP;
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for (;;) {
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if (poll(pfds, 2, -1) < 0) {
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if (errno != EINTR) suicide("poll failed");
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}
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if (pfds[0].revents & POLLIN) {
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process_client_socket();
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}
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if (pfds[0].revents & (POLLHUP|POLLERR|POLLRDHUP)) {
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suicide("scriptdSock closed unexpectedly");
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}
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if (pfds[1].revents & (POLLHUP|POLLERR|POLLRDHUP)) {
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exit(EXIT_SUCCESS);
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}
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}
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}
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static void signal_handler(int signo)
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{
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switch (signo) {
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case SIGCHLD:
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while (waitpid(-1, NULL, WNOHANG) > 0);
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break;
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case SIGINT:
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case SIGTERM: _exit(EXIT_FAILURE); break;
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default: break;
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}
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}
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static void setup_signals_scriptd(void)
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{
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static const int ss[] = {
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SIGCHLD, SIGINT, SIGTERM, SIGKILL
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};
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sigset_t mask;
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if (sigprocmask(0, 0, &mask) < 0)
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suicide("sigprocmask failed");
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for (int i = 0; ss[i] != SIGKILL; ++i)
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if (sigdelset(&mask, ss[i]))
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suicide("sigdelset failed");
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if (sigaddset(&mask, SIGPIPE))
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suicide("sigaddset failed");
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if (sigprocmask(SIG_SETMASK, &mask, (sigset_t *)0) < 0)
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suicide("sigprocmask failed");
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struct sigaction sa = {
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.sa_handler = signal_handler,
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.sa_flags = SA_RESTART|SA_NOCLDWAIT,
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};
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if (sigemptyset(&sa.sa_mask))
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suicide("sigemptyset failed");
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for (int i = 0; ss[i] != SIGKILL; ++i)
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if (sigaction(ss[i], &sa, NULL))
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suicide("sigaction failed");
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}
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void scriptd_main(void)
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{
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assert(valid_script_file);
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prctl(PR_SET_NAME, "ndhc: scriptd");
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umask(077);
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setup_signals_scriptd();
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do_scriptd_work();
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}
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